In a steam turbine as an example of turbo-type fluid machines, fluid composed of high-temperature and high-pressure vapor is used as a working fluid to rotate a turbine rotor at high speed. A stator is arranged to face the outer circumferential surface of the turbine rotor. A gap is provided between the outer circumferential surface of the turbine rotor and the inner circumferential surface of the stator so as not to contact the Inner circumferential surface of the stator when the turbine rotor rotates.
Since the fluid leaked through this gap is not used to rotate and drive the turbine rotor, a sealing device is provided to restrain the fluid leaked from the gap as much as possible.
The sealing device has many noncontact-type sealing fins arranged on the outer circumferential surface of the turbine rotor or on the inner circumferential surface of the stator. The gap between the leading edges of the sealing fins and the surface facing thereto is narrowed as much as possible to reduce flow volume of the leakage fluid.
However, fluid leakage is still caused even when the sealing device is provided. When leakage flow velocity in the sealing device is displaced in the radial direction while keeping circumferential components, pressure distribution in the sealing device is unbalanced in the circumferential direction, causing fluid force destabilizing the turbine rotor. This destabilizing fluid force is caused mainly by the swirling flow components on the upstream side of the sealing device.
Destabilizing force is increased as swirling flow components are increased, which easily causes unstable self-excited vibration of the turbine rotor. This is the fact based on experiences, model tests and fluid analysis programs, and it is made clear that reducing the swirling flow components on the upstream side of the sealing device is effective in restraining self-excited vibration of the shaft. Such self-excited vibration is known as “steam whirl” in the field of steam turbines, and this is a problem involved with the increase in steam pressure.